Transmitter-receiver system

ABSTRACT

A transmitter-receiver system is disclosed which may be used for remote control purposes. The transmitter has a radiated signal with a first frequency carrier modulated with a second frequency. A subcoder may be selectively plugged into the transmitter and has a third frequency which influences or modulates the radiated signal at a third frequency rate. The receiver of the system has means responsive to the first and second frequencies and also has a decoder selectively plugged into the receiver which has a disabling bias means and a third frequency responsive means. The disabling means normally prevents any signal from reaching the load of the receiver and the third frequency responsive means has an output when the third frequency is present which terminates the disabling means so that the receiver is enabled and a signal is passed to the receiver load. Without the decoder plugged into the receiver, the receiver is completely operable on the first and second frequencies to supply a signal to the load.

United States Patent I 1 1 9 Deming l l Aug. 21, 1973 i 1TRANSMITTER-RECEIVER SYSTEM [57] ABSTRACT [75] Inventor: Andrew F.Deming, Alliance, Ohi A transmitter-receiver system is disclosed whichmay [73] Assignee: The Alliance Manufacturing be used for remote controlpurposes. The transmitter pan), Inc. Alliance. Ohio has a radiatedsignal with a first frequency carrier modulated with a second frequencyA subcoder may be se- [22] Filed: Dec. 2, 1971 lectively plugged intothe transmitter and has a third fre uency which influences or modulatesthe radiated [211 App! 204l50 sigr 'ial at a third frequency rate. Thereceiver of the system has means responsive to the first and second fre-[52] U.S. Cl 325/37, 325/64, 340/171, quencies and also has a decoderselectively plugged 343/225 into the receiver which has a disabling biasmeans and [51] Int. Cl. H04b 7/00 a hird frequency responsive means. Thedisabling [58] Field of Search 325/37, 64.55, 392, means normallyprevents any signal from reaching the 325/393; 343/225, 228; 340/171 R,171 A load of the receiver and the third frequency responsive means hasan output when the third frequency is pres- [56] References Cited outwhich terminates the disabling means so that the UNITED STATES PATENTSreceiver is enabled and a signal is passed to the receiver 2,523,3158/1950 Mayle 325/37 x load l the decoder plugged h 2980.794 4/1961Hargreaves et a] l v 325/392 X the receiver is completely operable 0nthe first and sec- 3,366,961 l/l968 Goldstein 343/225 0nd frequencles toSupply a 518ml the load 3.522536 8/1970 Reynolds 343/225 X PrimaryExaminerBenedict V. Safourek Attorney-George V. Woodling, Louis V.Granger et al.

17 Claims, 7 Drawing Figures TRANSMITTER-RECEIVER SYSTEM BACKGROUND OFTHE INVENTION The disclosed transmitter-receiver system may be used inremote control systems, for example, a remotely controled garage dooropener. In such use the transmitter may be a hand-sized battery poweredlow output power transmitter complying with Federal CommunicationCommission regulations as to radiated power. The carrier may be in theVHF range, for example, with modulation in the audio or super-audiofrequencies.

The transmitter sends a signal to a corresponding receiver and if theproper carrier and modulation frequencies for that set of transmitterand receiver is received by the receiver, then an output signal isgiven. This output signal may be used to remotely control someparticular device,'for example, a garage door. In many cases the garagedoor being controlled is in a garage attached to the home and ifunauthorized persons were able to easily operate the garage dooroperator receiver, then unauthorized access to the garage and to thehome could be achieved. Thus a security problem occurs and it becomesincreasingly important to increase the number of codes and thecomplexity of the codes in order to prevent unauthorized access to thegarage and home. If there are only six different carrier frequencies andsix different modulation frequencies, then this gives a total of sixtimes six or 36 different possible codes. If there are carrierfrequencies and ten modulation, frequencies, for example, then thiswould give a total possibility of 100 codes. However because of FCCregulations, the number of carrier frequencies which may be used withoutinterference with each other is limited, thus limiting the totalpossible number of codes. Also, with only six or ten carrier frequenciesplus a similar range of modulation frequencies, it is relatively easyfor a law-breaker to gain access to the garage. For example, if such aperson had six or ten different transmitters each on one of the assignedcode of carrier frequencies, then each in turn could be turned on andgradually adjusted through the range of audio frequencies. Thus, all 36or 100 possible codes could be swept through in a matter of l or 2minutes and the lawbreaker could easily gain access to the garage orhome.

In many areas of high saturation of garage door operators, there is anincreasing problem of the transmitter of a neighbor operating the wronggarage door operator receiver. Thus the operator of an automobiledriving along a street and depressing the transmitter pushbutton switch,could trigger receivers to open garage doors, which are the wrong garagedoors, unless the carrier frequencies and modulation frequencies of thecoding scheme have sufficient separation therebetween, and do not have atendency to heterodyne to produce one of the carrier or modulationfrequencies of the coding scheme.

In order to make the garage and home more secure, more codes have beensuggested but this method of increasing the number of possible codes byincreasing the number of carrier or modulation frequencies, runs intodifficulty with the FCC regulations and runs into further difficultywith trying to select frequencies which do not interact with each otherby heterodyning so as to produce one of the frequencies of the codes.

One prior art attempt at increasing the security was to produce atransmitter and receiver system wherein the transmitter had one carrierfrequency out of a number of possible frequencies, for example, 6 or 10.Next, two separate modulation frequencies were provided in thetransmitter with the transmitter first emitting a radiated signal of thecarrier modulated by the first modulation frequency and then immediatelyafterward the first modulation frequency ceased and the secondmodulation frequency commenced for an additional time period. Thereceiver of that particular set would be tuned to that particularcarrier frequency and would have a detector means to detect the firstand second modulation frequencies with a time delay on drop out ofdetection of the first modulation frequency. This meant that the firstmodulation frequency had to be detected first, with a time delayhold-over of the relay contacts being held closed during the time thatthe second modulation frequency was detected, in order for an outputsignal to be developed by the receiver. This increased the security butrequired a considerably more complex receiver system and required a morecomplex transmitter system such that only the first and secondmodulation frequencies were transmitted and were transmitted in sequencebut not simultaneously.

A serviceman has an extremely difficult time to locate the cause ofspurious responses because the receiver is usually on 24 hours a day ona standby basis, awaiting the reception of a proper signal. The peculiarcircumstances which cause a spurious signal to trigger the receiver willnormally not occur when a serviceman is present and thus he seldom canfind the cause. This is different from a complete breakdown of theequipment where he can use a signal generator or other servicemansequipment to locate the break or short in the circuit and have itrepaired.

Another disadvantage with this prior art system is the increased numberof transmitters and receivers which must be manufactured and which mustbe stocked by a distributor. If there are ten different first carrierfrequencies and ten different second modulation frequencies plus tendifferent third modulation frequencies, this is 10 times 10 times 10 or1,000 different transmitters and receivers which must be manufacturedand also 1,000 different transmitters and receivers which must bestocked by the manufacturer and the distributor. This is sufficientlyburdensome that it becomes increasingly difficult to find a distributorwho is willing to stock all of these units.

Accordingly, an object of the invention is to provide atransmitter-receiver system obviating the abovementioned disadvantages.

Another object of the invention is to provide a transmitter subcodersuch that the transmitter simultaneously transmits three differentfrequencies.

Another object of the invention is to provide a transmitter systemwherein the security of a load actuated by a remotely controled receiveris materially increased.

Another object of the invention is to provide a transmitter subcoderwherein the subcoder does not detune the transmitter circuittransmitting the modulation frequency.

Another object of the invention is to provide a receiver-decoder whereinthe decoder does not detune the receiver circuit to its sensitivity to asecond modulating frequency.

Another object of the invention is to provide a transmitter subcoderwherein a third frequency is provided for only a short length of timeand then ceases to thus increase the security by requiring that thereceiver be responsive to this third frequency and then responsive tothe termination of the third frequency with only a modulated carrierwave.

Another object of the invention is to provide a transmitter system ofincreased security against spurious operation by requiring that thepower supply switch be closed, and the carrier frequency, modulationfrequency and subcoding frequency all be correct in order to transmit aproper signal which will operate a receiver system.

Another object of the invention is to provide a receivet-decoder whereinthe presence of a third frequency is detected on a conductor and thereceiver is disabled by a voltage condition on that same conductor.

Another object of the invention is to provide a receiver-decoder whichmay be plugged into an existing test point jack on an existing receiverwhich normally is used with only a first carrier and a second modulationfrequency.

Another object of the invention is to provide a transmitter-receiver setwhich permits a customer to purchase a simplified transmitter-receiveroperative on only first and second frequencies and later to add asubcoder and decoder with a third frequency response, if more securityis desired or if spurious operating signals are encountered.

SUMMARY OF THE INVENTION The invention may be incorporated in amodulated carrier system, comprising in combination, a transmitter and areceiver; said transmitter comprising output circuit means, means todevelop a first frequency carrier in said output circuit means, means todevelop a modulation frequency, and means connecting said modulationfrequency developing means to said output circuit means to establish acarrier wave output from the transmitter influenced at said modulationfrequency rate for a first time period and to terminate said modulationfrequency thereafter; said receiver comprising first means responsive tosaid first frequency, a main load, disabling means connected to disablesaid main load and operative in the absence of areceived signalcontaining said modulation frequency, and means responsive to saidmodulation frequency and having an output connected to terminate saiddisabling means output for at least a second time period to enable saidreceiver to thus pass current to said main load upon the presence of areceived signal containing said modulation frequency and subsequentlysaid first frequency.

Other objects and a fuller understanding of the invention may be had byreferring to the following description and claims, taken in conjunctionwith the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of atransmitter circuit;

FIG. 2 is a schematic diagram of a subcoder connectable to thetransmitter of FIG. 1;

FIG. 3 is a schematic diagram of a modified form of subcoder;

FIG. 4 is a schematic diagram of a main receiver responsive to first andsecond frequencies;

FIG. 5 is a schematic diagram of a receiver decoder which may beconnected to the receiver of FIG. 4 and which adds a third frequencycapability;

FIG. 6 is a schematic diagram of an alternative receiver decoder whichmay be electrically connected to the main receiver of FIG. 4; and,

FIG. 7 is a graph of voltages obtainable in the circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a schematic diagram ofa transmitter 11 which incorporates the invention. This transmitter hasa means 12 to develop a carrier frequency and this is shown as a carrierfrequency oscillator. The transmitter 11 also has a means 13 to developa modulation frequency and this means 13 is shown as a modulationfrequency oscillator. A power supply 14 is provided in the transmitterand this may be a primary battery, especially where the transmitter isof low power for example, a hand-sized VHF transmitter usable withremote control of garage door operator receivers. A switch such as apush-button switch 15 is provided as is output circuit means 16. Meansis provided including the switch 15 to connect the power supply 14 tothe frequency developing means 12 and 13 to establish an output from theoutput circuit means 16 which contains both the carrier and modulationfrequencies. To this end, the carrier frequency oscillator 12 includes atransistor 20 having an emitter 21 connected through a jumper 22 and anoutput load impedance shown as an output load resistor 23. The resistor23 is connected in the output circuit 16 and this output circuit meansmay include a parallel resonant circuit of a capacitor 25 and inductance26. The inductance may have a movable permeable core 27 for tuningpurposes. Capacitors 29 and 30 connect the lower end of the parallelresonant circuit 25-26 to the base 31 of transistor 20. These capacitorsprovide a feedback from the tank circuit 25-26 in order to sustainoscillations. The upper end of this tank circuit is connected to thecollector 32 of the transistor 20 in order to complete the outputcircuit means 16.

The power supply 14 has first and second terminals 34 and 35,respectively, of different voltages. The first terminal 34 is thepositive terminal of the power supply 14 and is connected to a conductor36 and through a current limiting resistor 37 to the base 31 oftransistor 20. The second power supply terminal is connected through thepush-button switch 15 to a conductor 38 and this may be considered theground side of the power supply 14. This conductor 38 is connectedthrough a bias resistor 39 to the base 31 of transistor 20. Conductor 38is also connected to the interconnection of capacitors 2Q-30 andresistor 23. The transistor 20 with the connections as shown willoscillate at a frequency determined by the parallel resonant circuit25-26 which may be in the VHF range, for example, 250-300 MHz.

The modulation frequency oscillator has a circuit quite similar to thatof the carrier frequency oscillator except with different values ofcomponents to have the modulation frequency lower than the carrier fre--quency, for example, in an audio or super-audio range of SOD-20,000 Hz.The modulation frequency oscillator 13 includes a transistor 40 with anemitter 41 connected through a resistor 43 to the conductor 38. Aparallel resonant tank circuit is provided in this modulation frequencyoscillator 13 including a capacitor 45 and inductance 46. The inductance46 may have a movable permeable core 47 for tuning to the desiredmodulation frequency. A feedback capacitor 50 connects the lower end ofthe tank circuit 45-46 to the base 51 of transistor 40. The upper end ofthe tank circuit 45-46 is connected to the collector 52 by a conductor53 which also connects together the tank circuits of the two oscillators12 and 13. A current limiting resistor 57 connects the conductor 36 tothe base 51 and a resistor 59 connects the base 51 to the conductor 38.The transistor 40 will oscillate at the modulation frequency determinedby the values of the parallel resonant circuit 45-46. A power supplycapacitor 58 may be directed across the power supply 14.

First, second and third junctions 61, 62 and 63, respectively, areprovided in the transmitter 11. Junction 61 is connected to theinterconnection of emitter 21 and jumper 22. The second junction 62 isconnected to the ground conductor 38 and the third junction 63 isconnected to the positive power supply terminal 34 via conductor 36.These junctions 61-63 provide a ready means for connection to a subcoder66 shown in FIG. 2. This subcoder 66, together with the transmitter 11,has a means 68 to develop a third frequency. This third frequency may bea subcode and preferably is a frequency lower than either the carrier orthe modulation frequency. The third frequency developing means 68 is atleast partly in the subcoder 66 and in this preferred embodiment isshown as being incorporated in circuitry of the subcoder 66. Thesubcoder 66 has first, second and third terminals or connectors 71, 72and 73. These connectors are connectable to the junctions 61-63,respectively, and for ease of this interconnection the subcoder 66 maysimply be plugged into the transmitter 11 by having male connectors71-73 on a terminal strip 74 receivable in female connections of thejunctions 61-63. The transmitter 11 may be mounted on a printed circuitboard as an example, and the subcoder may be mounted on another smallerprinted circuit board with the terminal strip 74 an integral partthereof. This subcoder 66 in the preferred embodiment of FIG. 2 includesa Darlington transistor pair 75 connected to null resonant circuit means76 to act as an oscillator which oscillator may be the principalcomponent of the third frequency developing means 68. The circuit meansmay take one of several forms and in FIG. 2 is shown as including abridge T filter network. Resistors 77, 78 and capacitor 79 form one Tand capacitors 80, 81 and variable resistor 82 form another T whichtogether form the bridge T 76. This bridge T network 76 has terminals 83and 84. A feedback capacitor 85 establishes the transistor 75oscillating at the null frequency of the bridge T circuit 76. A biasresistor 86 biases the transistor 75 into a proper operating condition.

The second connector 72 is connected to a ground conductor 88 and thirdconnector 73 is connected to the positive power supply voltage intransmitter 11 and is connected to a conductor 89 in the subcoder 66.This conductor 89 is connected through a resistor 90 to the terminal 83of the null resonant circuit means 76. This .null resonant circuit means76 is one which has a null at the desired frequency, hence a minimumoutput across terminals 84 and conductor 88. The output at terminal 83is passed by a coupling capacitor 91 and resistor 92 to an outputcircuit which includes a transistor 94. The resistor 92 is connected tothe base 95 of this transistor 94. The emitter of transistor 95 isconnected to the connector 72 and the collector of this transistor isconnected through a current limiting resistor 96 to the connector 71.Accordingly, the conduction or non-conduction of transistor 94 gives anoutput signal on connectors 71 and 72.

A timing circuit 99 is provided in the subcoder 66 to provide a timedelay period. This may be considered a second time delay period with thefirst time delay period that established by the resonant circuit means76. Many such resonant circuit means take a certain finite time to ringor come up to full resonance. Such first time delay period may be quiteshort, for example, 0.01 seconds up to 0.1 seconds. The timing circuit99 includes primarily a capacitor 100 and a transistor 104 to amplifythe effect of capacitor 100. Resistors 102, 103 cooperate with capacitor100 for an RC charging time delay network. This time delay may be anysuitable value for example, 1/10 second to three seconds and after thecapacitor 100 is charged, then the transistor 104 is turned oncontinuously. This turns on transistor 94 continuously for a minimumpotential difference across terminals 71 and 72. This is after thesecond time delay period of perhaps z second and during that second timedelay period, while capacitor 100 is charging, the transistor 20 isinfluenced at the third frequency rate by the output from Darlingtontransistor pair 75 appearing at terminal 83. This means that during thissecond time delay period the transistor 94 is turned on and off at thethird freguency rate. When transistor 94 is not conducting, this meansthere is a high impedance condition between terminal 71 and 72. Thisturn on and off of transistor 94 interrupts the radiated modulatedcarrier at the third frequency rate. This is like 100% modulation with asquare wave.

OPERATION Now referring to FIG. 1, it may be observed how the subcoder66 affects the transmitter 11. When the connectors 71-73 are pluggedinto the junctions 61-63, then the interconnection of connector 72 andjunction 62 establishes a reference potential in the subcoder 66. Thisis the 0 volts or ground reference potential. The interconnectionofjunction 63 and connector 73 establishes another potential in thesubcoder 66 at a potential different from that on terminal 72.Accordingly, an operating voltage is supplied to this subcoder 66. Inthe example shown this is plus nine volts applied to the subcoder 66.The interconnection ofjunction 61 and connector 71 establishes that theoutput of the subcoder 66 is applied to the transmitter 11. Moreparticularly, the output of the subcoder 66 appears on connectors 71 and72 and it will be seen in FIG. 1 that this output is applied tojunctions 61 and 62 which is in parallel with the output load resistor23. The jumper 22 may easily be formed from a U-shaped bend in the leadof this resistor as it is mounted on the printed circuit board. Thisohms, resistor 96 was 470 ohms and transistor 94 when conducting had thedifference of about 90 ohms impedance. Accordingly, it will be seen thatthe transmitter 1 1 operation is virtually unaffected in its operationduring the time transistor 94 is conducting, because there are nochanges in impedance or circuit parameters. Thus, as the transistor 94intermittently conducts at the third frequency or subcoding rate, thisestablishes the influence on the transmitter 11 at this third frequencyrate. More specifically, the output circuit means 16 of this transmitter11 will radiate a modulated carrier wave interrupted at the thirdfrequency rate. In one actual embodiment of transmitter made inaccordance with this invention, this third frequency was on the order of300 to 15,000 Hz. The radiation is from the inductance 26 which acts asa radiating antenna.

The timing circuit 99 establishes the charging of capacitor 100 from thepower supply source 14. This is the second time delay period and thismight be 1! 10 to 3 seconds, for example. After this second time period,the capacitor 100 is charged, which means that transistor 104 is turnedfully on and this turns transistor 94 fully on. Accordingly, it is nolonger influenced by the output from the oscillator 68. Also thiscontinuous conduction of transistor 94 means that the transmitter 11 isno longer influenced at the third frequency rate. More specifically, thecontinuous conduction of transistor 94 means that the carrier wave istransmitted as a modulated carrier wave modulated only at themo'dulation frequency of oscillator 13 and is not influenced at anythird frequency rate. This has the advantage that it does not detune themodulation frequency oscillator and hence the receiver of thetransmitter-receiver set will be receiving a modulation frequency and acarrier frequency at the proper values.

SECOND EMBODIMENT FIG. 3 shows an alternative subcoder 106 which may beused in place of the subcoder 66 of FIG. 2 and will also plug into thejunctions 61-63 in the transmitter of FIG. 1. To this end the subcoder106 again has the connectors 71-73 to be connected to the junctions61-63. This subcoder 106 has a means 108 to develop a third frequencywhich includes a transistor 109 and resonant circuit means 110 shown asa tuning fork. This may be any of the usual forms of tuning forkoscillator circuits, with a capacitive plate 1 l2 cooperating with thetuning fork 110 and supplying drive to the base of transistor 109.Another capacitive plate 1 13 cooperating with the tuning fork has afeedback from the output of a transistor 114 to sustain oscillation. Theoscillation of transistor 109 is supplied to an emitter followerresistor 115 and this output is passed by a coupling capacitor 116 tothe base input of transistor 114. The output of transistor 114 appearsat the collector for the aforementioned feedback and is coupled throughanother coupling capacitor 117 to the base 95 of transistor 94. Again atiming circuit 99A is provided which includes transistor 104, resistor103 and capacitor 100.

OPERATION When the subcoder 106 is plugged into the transmitter 11 itoperates in essentially the same manner as when subcoder 66 was pluggedinto transmitter 11. A first time delay period is established afterpush-button switch 15 is closed. This first time delay period is causedby the tuning fork 110 or resonant circuit means building up theamplitude of oscillations to the normal value. This might be l/ 100 to Aof a second. The timing circuit 99A establishes a second time delayperiod during which the modulated carrier wave being radiated isinfluenced at the third frequency rate. During this second time delayperiod, the capacitor 100 is charging and also during this time theoscillator 108 is oscillating and affecting the base of transistor 94 atthis third frequency rate. Accordingly, transistor 94 is turned on andoff at this third frequency rate which turns on and off the modulatedcarrier frequency radiated from the output circuit means 16 at thisthird frequency rate. At the completion of the second time delay period,the capacitor is virtually charged which means that transistor 104 isturned fully on as is transistor 94, hence it is no longer influenced bythe continuously running oscillator 108. Accordingly, after this secondtime delay period the radiated emissions are only of the carriermodulated at the modulation frequency of oscillator 13. The transistor104 is an amplifier and also a buffer to prevent the continuousconduction of transistor 94, subsequent to the second time delay period,from influencing the oscillator 108. This has the advantage of notaffecting the frequency of the oscillator circuit 108 and hencemaintaining the same frequency in a particular transmitter-receiver set.

From the above it will be noted that either subcoder 66 or 106 may beused interchangeably with the transmitter 11 and prior to plugging asubcoder into the transmitter, the transmitter is a completely operableunit radiating a modulated carrier wave and usable with a receiver tunedto the same carrier and modulation frequencies. lf security is requiredin addition to that afforded by the possible carrier frequencies andpossible modulation frequencies, then the subcoder 66 or 106 may easilybe added to the transmitter and a complementary decoder added to thereceiver. For example, if 10 possible transmitter frequencies are usableand 10 possible modulation frequencies are usable, this would give 100possible codes. Adding a subcoder with another 10 possible frequencies,this gives 1,000 possible codes. Actually it has been found that thesecurity achieved by the addition of this third frequency isconsiderably more than merely a 10-fold increase in security. Referringto FIG. 3 with the tuning fork 1 10, it will be observed that thistuning fork could be induced into oscillation by a physical shock.However, this alone does not establish a third frequency output. Beforethe right combination of carrier, modulation and subcoding frequenciesoccur, five things must be properly established:

l. The push-button switch 15 must be closed; 2. The carrier frequencyoscillator 12 must be at the right frequency; 3. The modulationfrequency oscillator 13 must be at the right frequency; 4. The thirdfrequency oscillator 108 must be at the right frequency; and, 5. Thecapacitor 100 must not be charged. This fifth criteria above isaccomplished by the timing circuit 99 and takes only l/lO to threeseconds to accomplish. Accordingly, a lawbreaker would have a very shorttime in order to try to fulfill these five criteria. This is why thesecurity is increased much more than lO-fold by the addition of a thirdfrequency. FIG. 4 illustrates a preferred embodiment of the receiver 211 incorporated in the transmitter-receiver system. The radio receiver211 is adapted to be operative on a received signal of a predeterminedfirst frequency carrier modulated by a lower second modulationfrequency, and also subject to receiving random noise signals. Thereceiver 21 1 includes a receiving antenna 212 supplying an input to atransistor 213 which is an isolation stage. The signal is then passed toa superregenerative circuit 214 which includes a transistor 215 with aparallel resonant output circuit 216. This parallel resonant outputcircuit 216 is tuned to a predetermined first frequency carrier, whichfor example, might be in the order of 250 MHz. The output of thesuperregenerative circuit 214 appears at a terminal 217 and contains thecarrier frequency, the modulation frequency and also a squelch frequencyintermediate these two frequencies. In this example, the modulationsecond frequency may be an audio or super-audio frequency.

The squelch frequency depends upon the constants of the circuit 214 andmay be 600 KHZ, for example. This output is applied to resistors 218 and219 in series and to a capacitor 220 which presents a low impedance toground for the squelch frequency and accordingly the modulationfrequency signal is passed by a capacitor 222 to the transistor 213 in areflex circuit for amplification of such modulation frequency signal.This amplified output appears across a capacitor 223 and it is passed bya coupling capacitor 224 to an input terminal 225 of a lower frequencyamplifier 227. The amplifier 227 is shown as a transistor supplying anoutput to a detector circuit which includes a tuned load 229 and anuntuned load 230.

The parallel resonant circuit 216 is tuned to resonance to the first orcarrier frequency and hence is a first means responsive to this firstfrequency. The tuned load 229 is tuned to resonance at the second ormodulation frequency and hence is a second means responsive to thissecond frequency. The tuned load 229 includes a transform 231 with amovable slug core 232 and the transformer has a primary and a secondarywinding 233 and 234, respectively with the primary winding connected tothe output of the transistor 227. The output from the transistor 227 isthrough the pri mary winding 233, a capacitor 235 and a resistor 236 toground. Each of the output signals of the transistor 227 appear acrossthe resistor 236, and accordingly, a center conductor 238 of a testpoint jack is connected to this junction of the capacitor 235 andresistor 236. The outer conductor 239 of this test point jack isconnected to ground. With all of the audio signals including the randomnoise appearing across resistor 236, this may be considered the input tothe untuned load 230 and it is stated as being untuned to distinguish itfrom the tuned load 229.

In this tuned load 229 capacitor 240 is connected across the secondarywinding 234 to tune it to resonance which is reflected into the primarywinding 233. The upper terminal of capacitor 240 is a first terminal 241relative to ground 242 which may be considered a second terminal. Thelower end of capacitor 240 is connected to a junction 243 at the untunedload 230 and passes from there through a resistor 244 to ground. Acapacitor 245 is connected in parallel with resistor 244 and a diode 246is connected with a polarity to conduct current from junction 243 to thetest point center conductor 238.

A unidirectional conducting device shown as a diode 248 is connected tothe first terminal 241 to pass current to a main load 249. This mainload includes a transistor 250, a relay 251 and a time delay capacitor252. A diode 253 connects the capacitor 252 between diode 248 and ground242. A discharge resistor 254 is connected across the capacitor 252.When the time delay capacitor has been sufficiently charged positive,then current is passed through a resistor 255 to the base of transistor250. This occurs when the voltage of the capacitor 252 exceeds theforward voltage drop from base to emitter of the transistor 250.

A power supply 258 is provided such that when a switch 259 is closed, astep-down transformer 260 is energized and the secondary thereofenergizes first and second terminals 261 and 262 of a terminal strip 264which also has a third terminal 263. The AC voltage between terminals261 and 262 is supplied through a rectifier 265 to a filter capacitor266 so that a supply conductor 267 is positive relative to ground 242.The relay 251 is connected to this DC supply conductor 267. The relaycontrols single pole double-throw contacts with a contact blade 268connected to ground 242. The normally closed contacts 270 are connectedto a terminal 271 at the junction of first and second bleeder resistors272 and 273. A capacitor 275 is connected in series with resistor 273between terminal 271 and a terminal 276, which is connected to the baseof transistor 250. The normally open contacts 276 are connected by alead 277 to the third terminal 263 on the terminal strip 264. Thisprovides an external connection controled by the energized orde-energized condition of the load 249. Protective capacitors 278 and279 are connected between terminal 271 and ground and terminal 276 andground and a filter capacitor 280 is connected across the relay 251. Aprotective diode 281 is also connected across this relay 251.

OPERATION The radio receiver 211 conveniently may be used in a remotecontrol of a physical device, for example, the remote control of agarage door operator from a low powered transmitter. As the userapproaches the garage in driving his automobile, he presses a button onthe transmitter to place it in operation. The transmitter emits a signalwhich is a selected one of a plurality of carrier first frequencies andone ofa plurality of second modulation frequencies. As explained above,it may also contain a third lower frequency, however, the main receiveras described so far in FIG. 1 is usable with just a singly modulatedcarrier frequency. The tuned circuit 216 is responsive to the firstfrequency and if the received signal on the antenna 212 is one having acarrier at this frequency, then the signal is passed to the lowerfrequency amplifier 227. Normally noise being received on the antenna212 is passed and is further amplified by the amplifier 227. The untunedload 230 is that which is more responsive to this noise andoff-frequency signals than the tuned load 229, for example, there may befour times as much voltage across resistor 236 as across the primarywinding 233 due to this noise. The polarity of the diode 246 is suchthat junction 243 will be negative as caused by the audio frequencynoise on this resistor 236. In a practical circuit this may be 1 to 3volts negative at junction 243 relative to ground 242. The conduction bydiode 246 makes the test jack conductor 238 positive by a small amountdue to this rectification of the audio frequency noise.

When a received signal of the proper first and second frequencies isreceived, then it is passed by the superregenerative circuit 214 and thetuned load 229 resonates at this second frequency. This means a highvoltage appears across the secondary winding 234 and hence across theprimary winding 233. Diode 248 is poled to conduct when the firstterminal 241 is positive. However, it will be noted that diodes 246 and248 are in effect poled in opposite directions. This means that theoutputs of the tuned and untuned loads 229 and 230 are effectivelyconnected in opposition. The output of the untuned load 230 appearsacross terminal 243 and 242 and is negative on terminal 243 relative toground terminal 242. The output of the tuned load 229 is positive on thefirst terminal 241 relative to the ground 242. Accordingly, under normalstand-by conditions, the output from the noise from the untuned load 230predominates and no current is passed to the main load 249. When theproper second frequency signal is received however, then the output fromthe tuned load 229 at terminal 241 predominates and exceeds the negativevoltage at terminal 243. Under this condition the output of the tunedload 229 is more positive than a given value to pass current to the mainload. 249. The given value in this case is a voltage, for example,2/l0ths of a volt to cause the diode 248 to conduct. Upon conduction thetime delay capacitor 252 will be charged. At the threshold of receivedsignal, diode 248 conducts only on the crests of the positive halfcycles but as the received signals grow stronger, the diode 248 mayconduct for practically the entire positive half cycles. Resistor 254 isconnected to continually discharge capacitor 252 but at some point thecharge on capacitor 252 will reach a voltage value exceeding the forwardbias on transistor 250. This may be 7/l0ths of a volt, for example, forsilicon transistors and hence transistor 250 will conduct to energizerelay 251. When this happens the normally closed contact 270 is openedand the normally open contact 276 is closed. This places an outputsignal on the third terminal 263, which may be used for any desiredpurpose, for example, in a garage door operator this may be used tocontrol a power relay energizing a motor which drives the garage door.The operation of this particular part of the circuit is more fullydescribed in my previous [1.5. Pat. No. 3,579,240.

FIG. shows a preferred embodiment of a decoder 290 having a test plug291 which may be plugged into the test jack 238-239 of the main receiver211 to add a third frequency capability thereto. The decoder 290 may bemade on a printed circuit board and may be practically small andlightweight to support itself physically when plugged into the testpoint jack 238-239. Electrical connections are also made at the sametime of this physical support. The test plug 291 includes a centerconductor 292 connecting to the center conductor 238 and an outerconductpr 293 connecting to the outer conductor 239 of the test jack.The outer conductor 293 is connected to an internal ground 294 of thedecoder 290.

The decoder 290 includes generally a disabling means 296 and a thirdfrequency responsive means 297. The disabling means 296 may also beconsidered a bias means to bias the main receiver 211 so that no currentis passed to the main load 249. The third frequency responsive means 297may also be considered a tuned circuit resonant to the third frequency,which is lower than the carrier frequency. The decoder 290 includes apower supply means 299 including a rectifier 300 poled to conductcurrent through a flexible lead 301 to the first terminal 261 on theterminal strip 264. This flexible lead 301 may readily be connected onthe terminal 261 when the decoder is plugged into the test jack. Thepower supply 299 also includes a filter capacitor 302 connected betweenthe voltage supply conductor 303 and ground 294. This polarity of therectifier 300 makes supply conductor 303 negative relative to ground294, for example, -3l volts. The ground 294 of decoder 290 is connectedto the ground 242 of the main receiver and with the first terminal 261connection, this provides an operating voltage to the decoder 290 andalso provides a reference potential; namely, ground.

The test plug center conductor 292 is connected to a terminal 305 andthe disabling means 296 is connected between this terminal and thesupply conductor 303. This disabling means 296 includes generally avoltage dropping resistor 306 and a transistor 307. A bias resistor 308is connected between ground 294 and the base of transistor 307 and sinceground is positive of the supply conductor 303 in this decoder 290, thisbiases the transistor 307 normally into conduction. The circuit may betraced considering FIG. 5 in conjunction with FIG. 4. Starting withground 294 or 242, which is positive, the current flows upwardly throughresistor 244, the diode 246, the test point center conductor 238, testplug center conductor 292, down through resistor 306 and transistor 307to the negative supply conductor 303. This current flow makes the upperend of resistor 244 at junction 243 negative with respect to ground. Ina practical circuit constructed in accordance with this invention, thiswas made to be about 20 volts negative relative to ground. This issufficient negative bias voltage supplied by the biasing or disablingcircuit 296 such that the main receiver is disabled. By this is meantthat no current may be passed to the main load 249. Thesuper-regenerative circuit 214 is a senstive circuit with a gain in theorder of one million, yet irrespective of the strength of the signal onthe antenna 12, for example, even if the proper first and secondfrequencies are present there will not be sufficient positive voltage atterminal 241 relative to ground 242 such that the output of the tunedload 229 can overcome this negative voltage output of the untuned load230 plus the bias from the decoder 290. This is why the circuit isdescribed as being disabled by the negative bias supply from the decoder290.

The decoder 290 also. includes the third frequency responsive means 297which in this preferred embodiment is a tuned circuit resonant to thisthird frequency. The tuned circuit includes an inductance 310 with atunable core 311. A capacitor 312 is connected in parallel with theinductance 310 for parallel resonance. This circuit is tuned toresonance at the third frequency which is lower than the first orcarrier frequency and in this preferred embodiment is also lower thanthe second or modulation frequency. For example, in one practicalcircuit this might be in the range of 500 to 5,000 Hz. Upon parallelresonance the voltage across this inductance 310 rises and thisestablishes turn-off of the transistor 307 to terminate the negativebias on the junction 243 and thus enable the receiver 211. Since thereceiver is at that time enabled, this means that if the signal isreceived containing the proper first and second frequencies, thencurrent is passed to the main load 249 at that time.

The third frequency responsive means 297 includes in this preferredembodiment a buffer amplifier 313 shown as a Darlington transistor pair.Power is supplied to this amplifier through a resistor 314 from thepositive operating voltage which in this case is ground 294. The inputbase of the Darlington pair is connected by a coupling capacitor 315 tothe upper end of the tank circuit 310312. The emitter output of theDarlington pair is fed through a resistor 316 to the negative operatingvoltage at conductor 303. The AC output of the Darlington pair issupplied through a coupling capacitor 317 to the input base of a drivertransistor 320. A high impedance isolating resistor 322 connects theinput terminal 305 to the upper end of the tank circuit 310-312 to havesupplied thereto the low frequency signals present on the test jackcenter conductor 238. An accelerator circuit 328 is provided in thesubcoder or decoder 290. This accelerator circuit includes a diode 329connected from the lower end of resistor 306 to a junction 330 betweenvoltage divider resistors 331 and 332 connected between ground and thenegative supply conductor 303. A resistor 333 and a capacitor 334 areconnected in series between junction 330 and ground. A resistor 335 andcapacitor 336 are connected in series between the junction of resistor333 and capacitor 334 and the base of the transistor 320. A resistor 337is connected from the base of transistor 320 to conductor 303.

OPERATION When a signal is received, correct in the first frequency,this is passed by the superregenerative circuit 214 and the tunedcircuit 216 thereof to the lower frequency amplifier 227. During normaloperation the voltage across resistor 236 is approximately four times asgreat as the voltage across the secondary 234. This is because the noiseand off-frequency signals generate a much larger output from the untunedload 230 than from the tuned load 229. However, during those periodswhen the correct second frequency is applied to the amplifier 227, thenthe parallel resonance of the detector circuit 229 assures that up toabout 90% of the total output of the detector appears across thesecondary winding 234 and only about across the resistor 236. Alsopresent across resistor 236, in this example, will be the aforementionedproper third frequency. This third frequency is applied to the decoder290.

During the initial period that the decoder 290 is powered, there will bea small leakage current through the high resistance 308 to charge thelarge capacitor 325 so that the base of transistor 307 is positiverelative to conductor 303 and transistor 307 is made conducting. Now,when the proper third frequency is passed along the center conductor 292of test plug 291, it will be passed to the third frequency responsivemeans 297. The voltage across the parallel resonant circuit 310-312 thusincreases considerably and the AC signals at this third frequency arepassed by the coupling capacitor 315 to the Darlington transistor pair313. This transistor pair has an output at the collector of the lasttransistor which is passed through the AC coupling capacitor 317 todrive the base of the transistor 320 at this third frequency rate. TheDarlington transistor pair 313 has a high impedance input to not loadthe inductance 310 and has a low impedance output to drive thetransistor 320. The tum-on of the transistor 320 on half wave positivepulses at the third frequency rate, rapidly discharges the capacitor325, perhaps in about 10 miliseconds.

FIG. 7 shows a graph of voltages available at different parts of thereceiver circuit 211. A curve 340 illustrates the signal received at theantenna 212 which as an example includes the proper first, second andthird frequencies between a time t and a time t Also a curve 341 showsthe time period of a received signal containing only the proper firstand second frequencies, the third frequency being missing. Prior to thistime t the voltage across the capacitor 325 is shown by a curve 342 andthis shows a voltage of 0.7 volts positive with respect to conductor 303across this capacitor 325. From time t to time 1,; namely, about 10miliseconds, the capacitor 325 discharges to have essentially zerovoltage thereacross at time t, as shown at a point 343 on this curve342. Prior to this the transistor 307 was conducting and this caused alarge DC negative bias voltage to appear on the center conductor testpoint 238. In the aforementioned circuit this might be a -l9 volt DCbias established on the curve 344 showing the voltage at this conductor238. During this same period prior to time t this large negative voltageat this conductor 238 causes conduction of the diode 246 so thatjunction 243 and first terminal 241 is at a minus voltage, for example,-1 8.8 volts as shown by curve 345 of the potential at this terminal241. Now at time t when the transistor 307 has stopped conducting, thepotential at test point 238 goes up to about +5 volts as shown at apoint 346. The reason for this positive voltage is that the thirdfrequency signal is now an off frequency signal as far as the tuned load229 is concerned. Accordingly, a large proportion of the total output oftransistor 227 appears across the resistor 236. This will be positive atthe center conductor 238. This positive voltage partially biases offdiode 246 so that the potential at the first terminal 241 remains atabout 4.8 volts.

The curves of FIG. 7 assume a condition wherein small hand-sizedtransmitters may be far away from the receiver and hence be emitting arelatively weak signal, not much more than the threshold of sensitivityof the receiver. This might be 5 microvolts of signal at the antenna212, for example. During this condition the very crests of the positivehalf cycles at the second frequency rate are resonated by the tunedcircuit 234-240 sufficiently so that these crests are passed by thediode 248. These charge the capacitor 252 relatively slowly and thiscapacitor is being continuously discharged by the paralleled resistor254. Under these conditions the charging of capacitor 252 may besufficiently slowly achieved, as shown by a curve 347, so that the plus0.7 volt charge condition on this capacitor 252 is not reached until atime which is subsequent to time The time I is that at which the thirdfrequency disappears from the received signal. When the third frequencydisappears, the noise, in this case an off frequency signal, on resistor236 decreases to a lower level, perhaps +2 volts as shown at a portion348 of the curve of voltage on this center conductor 238. During thissame time period from time t, to time t the voltage at terminal 241 isabout 4 to 5 volts negative because of conduction of diode 246 on thenegative half cycles of voltage on the resistor 236. This is shown by aportion 349 on the curve of the potential at terminal 241. This negativevoltage at terminal 241 makes it difficult for the proper secondfrequency to be passed by the diode 248 during positive half cycles.

Now at the time t when the third frequency has disappeared from theincoming received signals, this third frequency which is noise insofaras the tuned circuit 229 is concerned, has now disappeared and hence thenegative bias at terminal 241 has about disappeared as shown by aportion 350 of the curve of potential at this terminal 241. This meansthat the positive half cycles of the second frequency will be morereadily passed by diode 248 to more quickly charge capacitor 252 and 9hence the relay will be energized at a time by conduction of transistor250. This relay energization is shown by a curve 351 as occurring at thetime t;,. It will be understood that with a stronger signal containingfrequencies one, two and three, then the energization and pullin of therelay 251 may occur prior in time to the time namely, prior to the timewhen the third frequency disappears from the incoming signal.

At the time I, when the third frequency has ceased, the decoder 290 willbe conditioned so that the third frequency voltage across the tankcircuit 310-312 disappears. This causes transistor 320 to ceaseconduction and hence the capacitor 325 starts to charge as shown byportion 352 of the curve of voltage across this capacitor. This chargeis relatively slow because of the high resistance of resistor 308. Atsome point in time not necessarily related to the time t whenfrequencies one and two cease, the capacitor 325 will charge to about0.7 volts positive on the upper plate thereof so that transistor 307again starts to conduct. This establishes the aforementioned largenegative bias on center conductor 238 and on the terminal 241 as shownby portions 353 and 354 of the voltage curves on these terminals,respectively. This disables the receiver 211 so that no further signalsmay be passed to the relay 251. The capacitor 252 is now beingdischarged rather rapidly by resistor 254 as shown by curve portion 355and when the potential thereacross falls below 0.7 volts, the transistor250 ceases conduction and relay 251 drops out as shown by curve portion356. The above description is with the frequencies one and two ceasingat a time which is subsequent to the time However, should thefrequencies one and two cease prior to the time 2 then capacitor 252ceases charging and starts to discharge along a line 357. This wouldcause dropout of the relay ata point 358 on the relay energizationcurve.

The accelerator circuit 328 makes certain that once transistor 307starts to turn off, it actually does turn off and quickly. Resistors 331and 332 form a voltage divider, and the potential of junction 330 may behalf way between ground and conductor 303, for example, at a potentialofl5.5 V. At some time during turning off of transistor 307, thecollector thereof will rise in potential to a point exceeding l5.3volts, at which time diode 329 will conduct. This supplies a momentarycurrent through resistors 333 and 335 and capacitor 336 to help drivethe base of transistor 320 more positive and hence assure turn onthereof.

F IG. 6 shows an alternative decoder circuit 370 which may be used inplace of the decoder 290 and which has a test plug 371 with a centerconductor 372 and an outer conductor 373. This test plug 371 may beplugged into the test point jack 238-239 of the receiver 211. Thedecoder 370 has the same power supply 299 to establish a negativevoltage, for example, 31 volts on a negative power supply conductor 303.This is again established by a flexible conductor 301 which may beconnected to the terminal strip terminal 261. The power supply 299establishes this negative voltage on conductor 303 relative to a ground374 to which the outer conductor 373 of test plug 371 is connected. Thedecoder 370 includes generally a disabling means 376 and a thirdfrequency responsive means 377. The disabling meand 376 is quite similarto the disabling means 396 of the circuit of FIG. 5. This disablingmeans 376 includes a Darlington transistor pair 378 connected in serieswith a resistor 379 between the negative supply conductor 303 and aterminal 380 which is connected to the test plug center conductor 372.Normally, this transistor pair 378 is biased into conduction by the biasresistor 381. This bias resistor is a large value impedance and througha resistor 383 charges a capacitor 382 connected in series therewithbetween negative supply conductor 303 and the ground connection 374.Accordingly, normally during nonreceipt of third frequencies, thecapacitor 382 will be charged enough to bias transistor pair 378 intosaturation.

The third frequency responsive means 377 is a means tuned to beresponsive to this frequency and is shown in this embodiment as a tuningfork 385. This tuning fork may vibrate or be in resonance at theselected third frequency rate which again may be in the order of 300 to5,000 Hz. The input to the tuning fork 385 is from the terminal 380through a high resistance 393 and a coupling capacitor 386 and uponreceiving the proper third frequency this forces the tuning fork 385into vibration at its resonant frequency. The output from the tuningfork is from the other fork leg at a coupling capacitor 387 which drivesthe base of the transistor 388 at this third frequency rate. Thistransistor 388 is normally biased partly on by current through resistors389 and 394. Transistor 388 amplifies the third frequency voltage andthe output appears across the resistor 389. The third frequency rate ispassed by a capacitor 390 to a resistor 391. During the negativeexcursions of the upper end of resistor 391, a diode 392 will conduct,again at this third frequency rate. This rapidly drives the upper plateof capacitor 382 more negative so that the transistor pair 378 ceasesconduction. This stops the bias previously developed by conduction ofthis Darlington transistor pair. Accordingly, with the bias stopped,this enables the receiver 21 1 so that if the received signal on antenna212 contains a proper first and second frequencies, then the diode 248passes this second frequency output to energize the relay 251. This isessentially the same operation as the circuit of FIG. 5 and as describedby the aid of the curves of FIG. 7. Again when the third frequencyceases, the tuning fork 385 will cease oscillation. the transistor 388will cease amplifying, diode 392 will cease conduction and capacitor 382will again be permitted to charge slowly through the large valueresistor 381. When about 1.4 volts forward bias appears across capacitor382, this will again establish forward conduction of the transistor pair378 to again start the negative bias which disables the receiver 211.

The decoder 290 or 370 is responsive to the third frequency which may bereceived on the antenna 212 of the receiver 211. The decoder by itselfhas a terminal 305 or 380 which may be considered a first terminal,

ground 294 or 374 may be considered a second terminail and inputterminal 261 may be considered a third terminal of this decoder. Thebias means or disabling means 296 or 376 develops a bias voltage and thetransistor 307 or 378 is a switch means connected to this bias means toselectively connect and disconnect the bias voltage from the firstterminal 305 or 380. The frequency responsive means 297 or 377 isresponsive to a given frequency input; namely, the third frequency, onthis first terminal 305 or 380. In each decoder there is a meansconnecting the output of the frequency responsive means to the switchmeans to actuate this switch means to change the bias voltage conditionon the first terminal upon the incoming presence of the given or thirdfrequency. This connecting means includes the transistor 320 in FIG. andthe transistor 388 and diode 392 in FIG. 6. In the preferred embodimentof FIG. 5, this change of the bias voltage condition on the firstterminal is a disconnection of the bias voltage from this firstterminal. In both FIGS. 5 and 6 there is a power supply means with arectifier and filter so that the bias voltage is a DC voltage. The DCbias voltage appears between the first and second terminals 305 and 294and the second and third terminals 294 and 261 are adapted to have an ACvoltage applied thereto. The capacitor 325 or 382 provides a time delayof reapplying the bias voltage to terminal 305 or 380 upon cessation ofthird frequency input to said first terminal 305 or 380.

The time period 340 during which the first, second and third frequenciesare being transmitted, as shown in FIG. 7, is a first time period. Thethird frequency is terminated thereafter. in the receiver circuit thethird frequency responsive means is enabled during at least a secondtime period. This second time period is portions 349 and 350 of thevoltage curve on the terminal 241. The receiver is enabled during thisperiod, because the disabling bias means has been terminated during thistime.

The aforementioned receiver circuit 211 and decoders 290 or 370establish a circuit which accomplishes many objectives.

An additional advantage is achieved by having the subcoder 66 or 106 asa plug-in module rather than permanently wired into the transmitter 11.If a customer wants only a minimum security of one ofa range of carrierfrequencies and one of a range of modulation frequencies, then thetransmitter 11 is completely usable as part of a transmitter-receiverset. However, let us assume that after the consumer has purchased thetransmitter receiver set, he desires (1) either more security or (2)increased freedom from spurious interference which might be operatinghis garage door on spurious signals. In such a case, the serviceman ordealer may simply plug the subcoder into the transmitter 11, cut thejumper 22, place a similar decoder in the receiver of that set and thecustomer has accomplished both things; namely, increased security andincreased freedom from spurious signal operation of his garage door. Thetransmitter 11 at that time is one which has not only the twofrequencies originally built into it, but it also has the thirdfrequency of the subcoder.

Still another advantage is gained by the dealer or distributor becausehe does not need to stock nearly as many parts as he did before.Considering only the transmitter of the transmitter-receiver set, and ifone assumes ten possible carrier frequencies, possible modulationfrequencies and ten possible subcoding frequencies, the dealer ordistributor does not need to stock one thousand different transmitters.He needs to stock only ten different transmitters ofdifferent carrierfrequencies, plus ninety more transmitters for the ten differentmodulation frequencies of each of the ten carrier frequencies, plus tendifferent subcoders 66 or 106. This is a stocking of 1 l0 parts ratherthan l,0OO parts. Actually, the stocking of the additional transmittersto cover the possible codes of modulation and carrier frequencies, maybe eliminated if the dealer or distributor wishes to tune the movablecores 47 for the particular modulation frequency desired. These arecontinuously movable tuning slugs and with some frequency standard thesemodulation frequencies may quickly be set by a simple screwdriveradjustment. In such case, one would need to stock only 10 transmittersfor the 10 different carrier frequencies, plus 10 subcoders for the 10different subcoding frequencies for a total of stocking only twentyparts rather than one thousnad parts. A similar saving in the stockingof receivers of the transmitter-receiver set is also effected and hencethis is a tremendous saving in cost and convenience to the dealer anddistributor who needs to stock such a materially reduced number ofunits.

The decoder 290 or 370 easily may be plugged into an existing testpointjack 238 on any one of several existing receivers. Prior to theplug-in of this decoder, the receiver is completely operative on twofrequencies; namely, the first carrier frequency and the second ormodulation frequency. The test point jack is a valid test pointso that aserviceman in the field may readily check for proper carrier frequencyand proper modulation frequency. It is recognized that many servicemenin the field will not have complete laboratory test equipment, and infact, may have only a small DC voltmeter. Accordingly, the test point238 has been selected with this in mind. To test the proper operation ofthe receiver 211 in the field, the serviceman merely connects a DCvoltmeter from the test point center conductor 238 to ground 239 or 242.First, the modulation frequency is tuned considerably away from theproper point by moving the adjustable core slug 232. Next, the variablecapacitor in the tank circuit 216 is adjusted to get a maximum readingon the voltmeter. This will be because the super-regenerative circuit214 is passing a maximum of audio frequency signals, primarily noise,when the receiver carrier tuned circuit 216 is correctly tuned to thecarrier of the transmitted signal. Second, the modulation frequency isadjusted by the movable core slug 232 until the voltmeter gives aminimum reading. The reason why the voltmeter gives a minimum reading atthe proper modulation frequency is that the proportion of output voltagefrom the tuned load 229 relative to the untuned load 230 is a maximum atthe second or modulation frequency. Since the test point centerconductor 238 is effectively measuring the voltage output of the untunedload 230, this is then a minimum at the time that the second frequencyreceived signal is a maximum. Accordingly, it will be seen that thedecoder 290 or 370 plugs into an existing test point jack which is avalid and operable test point for determining the proper condition ofoperation of the receiver 211.

This plug-in to the test point jack establishes that the decoder 290 or370 is operable with a minimum of electrical connections to thereceiver. The plug-in establishes two electrical connections andadditionally provides physical support for the small lightweightdecoder. A third electrical connection by means of the flexibleconductor 301 is easily made to the terminal strip 264 by merely ascrewdriver to fasten the conductor lug to this terminal 261. Theseminimum electrical connections provide not only an operating voltage tothe decoder but also provide a reference potential in this case ground242 or 294.

The above description illustrates that the decoder 290 or 370 has threedifferent electrical conditions all on the same test point centerconductor 238: (l) the third frequency signal is supplied to the decoder290 or 370; (2) the entire reciever 211 is disabled by a bias voltageapplied on this center conductor; and, (3) the entire receiver 211 isenabled by a changed electrical condition on this semiconductor 238.

The present invention is also greatly advantageous to the user. Itenables the user to select one simplified system at a lower cost andlater if the location of this remote control receiver is in an areawherein spurious electrical disturbances are encountered and the garagedoor goes up and down undesirably and due to spurious electricaldisturbances, then the user may merely purchase an easily added decoder290 or 370 to convert his receiver into one responsive to threefrequencies rather than to only two frequencies.

Another important objective of the present invention is that it does notchange the effective band width of the receiver in order to add thethird frequency. One reason for this is that the third frequency remainson the incoming signal for only a short time, for example, a tenth of asecond from time t to 1 as shown in FIG. 7. As described above when thethird frequency is present, this acts generally as noise on the untunedload 230 to increase the signal thereof. This is shown at the portion349 of the voltage at the first terminal 241 on FIG. 7. This is anestablishment of a negative bias which means that the output from thetuned load 229 must be in excess of this bias in order to have currentpassed to the time delay capacitor 252. However, the existence of thisproper third frequency is received in the decoder which then terminatesthe bias; namely, the disabling means, and hence the receiver is enabledafter time without any change in bandwidth reception characteristics ofthe entire receiver 211.

The present disclosure includes that contained in the appended claims,as well as that of the foregoing description. Although this inventionhas been described in its preferred form with a certain degree ofparticularity, it is understood that the present disclosure of thepreferred form has been made only by way of example and that numerouschanges in the details of the circuit and the combination andarrangement of circuit elements may be resorted to without departingfrom the spirit and scope of the invention as hereinafter claimed.

What is claimed is:

1. A modulated carrier system, comprising in combination, a transmittcrand a receiver,

said transmitter comprising output circuit means,

means to develop a first frequency carrier in said output circuit means,

means to develop a modulation frequency,

and means connecting said modulation frequency developing means to saidoutput circuit means to establish a carrier wave output from thetransmitter influenced at said modulation frequency rate for a firsttime period and to terminate said modulation frequency thereafter;

said receiver comprising a main load,

first means responsive to said first frequency to pass a signal towardsaid main load,

disabling means having an output connected to said main load,

means responsive to said modulation frequency,

a conductor common to said disabling means and to said modulationfrequency responsive means, means establishing said modulation frequencyon said conductor,

means establishing a disabling voltage from said disabling means on saidconductor and operative in the absence of a received signal containingsaid modulation frequency to disable the signal from actuating said mainload,

and said means responsive to said modulation frequency having an outputconnected to terminate said disabling means output for at least a secondtime period to'enable said receiver.

2. A modulated carrier system as set forth in claim 1, wherein saidmodulation frequency responsive means establishes said second timeperiod overlapping said first time period.

3. A modulated carrier system as set forth in claim 1, wherein saidmodulation frequency developing means develops second and thirdfrequencies,

and said connecting means includes separable connections for said thirdfrequency developing means.

4. A modulated carrier system as set forth in claim 3, wherein saidmodulation frequency responsive means is responsive to said second andthird frequencies,

means permanently connecting said second frequency responsive means intosaid receiver to supply a signal to said main load in response to theproper first and second frequencies,

and means establishing said third frequency responsive means selectivelydisconnectable and connectable to the said receiver to establish saidreceiver operative to pass a signal to said main load upon the presenceof a received signal containing said first, second and third frequenciesand subsequently said first and second frequencies.

5. A modulated carrier system as set forth in claim 3, wherein saidmodulation frequency responsive means is responsive to said second andthird frequencies.

6. A modulated carrier system as set forth in claim 1, including meansestablishing at least part of said modulation frequency developing meansselectively connectable with said transmitter.

7. A modulated carrier system as set forth in claim 1, including meansestablishing at least part of said modulation frequency responsive meansselectively connectable with said receiver.

8. A modulated carrier system as set forth in claim 1, wherein saidmodulation frequency developing means develops second and thirdmodulation frequencies,

means permanently wiring said second frequency developing means intosaid transmitter to establish a modulated frequency output from saidoutput circuit means,

and means establishing plug-in connection of said third frequencydeveloping means to permit operation of said transmitter with only twofrequencies or selectively with three frequencies.

9. A modulated carrier system, comprising in combination, a transmitterand a receiver;

said transmitter comprising output circuit means,

means to develop a first frequency carrier in said output circuit means,and modulation frequency developing means selectively connectable tosaid output circuit means to establish a modulated carrier wave outputfrom the transmitter for a first time period and to terminate saidmodulation frequency thereafter; said receiver comprising a main load,first means responsive to said first frequency to pass a signal towardsaid main load,

modulation frequency responsive means and disabling means selectivelyconnectable to said main load with said disabling means operative in theabsence of a received signal containing said modulation frequency todisable the signal from actuating said main load,

a first connector as a part of said selectively connectable means,

means establishing said modulation frequency on said first connector,

means establishing a disabling voltage from said disabling means on saidfirst connector to disable the signal from actuating said main load,

and said modulation frequency responsive means having an outputconnected to terminate said disabling means output for at least a secondtime period to enable said receiver to thus pass current to said mainload upon the presence of a received signal containing said carrier andmodulation frequencies.

10. A modulated carrier system as set forth in claim 9, including meanssupplying an operating voltage from said receiver to said selectivelyconnectable modulation frequency responsive means and disabling means.

11. A modulated carrier system as set forth in claim 9, including meanssupplying operating voltages from said transmitter to said selectivelyconnectable modulation frequency developing means.

12. A modulated carrier system as set forth in claim 9, wherein saidmodulation frequency responsive means terminates said disabling voltageon said first connector to enable said receiver during said second timeperiod.

13. A modulated carrier system, comprising in combination, a transmitterand a receiver;

said transmitter comprising output circuit means,

means to develop a first frequency carrier in said output circuit means,means to develop in said output circuit means a second frequencymodulating said carrier frequency,

and third frequency developing means selectively connectable to saidoutput circuit means to establish a second frequency modulated carrierwave output from the transmitter influenced at said third frequency ratefor a first time period and to terminate said third frequencythereafter;

said receiver comprising a main load.

first means responsive to said first frequency to pass a signal towardsaid main load,

second means responsive to said second frequency to pass a signal towardsaid main load,

third frequency responsive means and disabling means having an outputand selectively connectable to said main load with said disabling meansoperative to disable said main load in the absence of a received signalcontaining said third frequency, and said third frequency responsivemeans having an output connected to terminate said disabling meansoutput for at least a second time period to enable said receiver to thuspass current to said main load upon the presence of a received signalcontaining said first, second and third frequencies and subsequentlysaid first and second frequencies.

14. A modulated carrier system as set forth in claim 13, wherein saidthird frequency developing means includes a bridge T filter tuned tosaid third frequency.

15. A modulated carrier system as set forth in claim 13, wherein saidthird frequency developing means includes a mechanically vibratabletuning fork.

16. A modulated carrier system as set forth in claim 13, wherein saidthird frequency responsive means includes a parallel resonant circuitresonant to said third frequency.

17. A modulated carrier system as set forth in claim 13, wherein saidthird frequency responsive means includes a mechanically vibratabletuning fork.

1. A modulated carrier system, comprising in combination, a transmitterand a receiver, said transmitter comprising output circuit means, meansto develop a first frequency carrier in said output circuit means, meansto develop a modulation frequency, and means connecting said modulationfrequency developing means to said output circuit means to establish acarrier wave output from the transmitter influenced at said modulationfrequency rate for a first time period and to terminate said modulationfrequency thereafter; said receiver comprising a main load, first meansresponsive to said first frequency to pass a signal toward said mainload, disabling means having an output connected to said main load,means responsive to said modulation frequency, a conductor common tosaid disabling means and to said modulation frequency responsive means,means establishing said modulation frequency on said conductor, meansestablishing a disabling voltage from said disabling means on saidconductor and operative in the absence of a received signal containingsaid modulation frequency to disable the signal from actuating said mainload, and said means responsive to said modulation frequency having anoutput connected to terminate said disabling means output for at least asecond time period to enable said receiver.
 2. A modulated carriersystem as set forth in claim 1, wherein said modulation frequencyresponsive means establishes said second time period overlapping saidfirst time period.
 3. A modulated carrier system as set forth in claim1, wherein said modulation frequency developing means develops secondand third frequencies, and said connecting means includes separableconnections for said third frequency developing means.
 4. A modulatedcarrier system as set forth in claim 3, wherein said modulationfrequency responsive means is responsive to said second and thirdfrequencies, means permanently connecting said second frequencyresponsive means into said receiver to supply a signal to said main loadin response to the proper first and second frequencies, and meansestablishing said third frequency responsive means selectivelydisconnectable and connectable to the said receiver to establish saidreceiver operative to pass a signal to said main load upon the presenceof a received signal containing said first, second and third frequenciesand subsequently said first and second frequencies.
 5. A modulatedcarrier system as set forth in claim 3, wherein said modulationfrequency responsive means is responsive to said second and thirdfrequencies.
 6. A modulated carrier system as set forth in claim 1,including means establishing at least part of said modulation frequencydeveloping means selectively connectable with said transmitter.
 7. Amodulated carrier system as set forth in claim 1, including meansestablishing at least part of said modulation frequency responsive meansselectively connectable with said receiver.
 8. A modulated carriersystem as set forth in claim 1, wherein said modulation frequencydeveloping means develops second and third modulation frequencies, meanspermanently wiring said second frequency developing means into saidtransmitter to establish a modulated frequency output from said outputcircuit means, and means establishing plug-in connection of said thirdfrequency developing means to permit operation of said transmitter withonly two frequencies or selectively with three frequencies.
 9. Amodulated carrier system, comprising in combination, a transmitter and areceiver; said transmitter comprising output circuit means, means todevelop a first frequency carrier in said output circuit means, andmodulation frequency developing means selectively connectable to saidoutput circuit means to establish a modulated carrier wave output fromthe transmitter for a first time period and to terminate said modulationfrequency thereafter; said receiver comprising a main load, first meansresponsive to said first frequency to pass a signal toward said mainload, modulation frequency responsive means and disabling meansselectively connectable to said main load with said disabling meansoperative in the absence of a received signal containing said modulationfrequency to disable the signal from actuating said main load, a firstconnector as a part of said selectively connectable means, meansestablishing said modulation frequency on said first connector, meansestablishing a disabling voltage from said disabling means on said firstconnector to disable the signal from actuating said main load, and saidmodulation frequency responsive means having an output connected toterminate said disabling means output for at least a second time periodto enable said receiver to thus pass current to said main load upon thepresence of a received signal containing said carrier and modulationfrequencies.
 10. A modulated carrier system as set forth in claim 9,including means supplying an operating voltage from said receiver tosaid selectively connectable modulation frequency responsive means anddisabling means.
 11. A modulated carrier system as set forth in claim 9,including means supplying operating voltages from said transmitter tosaid selectively connectable modulation frequency developing means. 12.A modulated carrier system as set forth in claim 9, wherein saidmodulation frequency responsive means terminates said disabling voltageon said first connector to enable said receiver during said second timeperiod.
 13. A modulated carrier system, comprising in combination, atransmitter and a receiver; said transmitter comprising output circuitmeans, means to develop a first frequency carrier in said output circuitmeans, means to develop in said output circuit means a second frequencymodulating said carrier frequency, and third frequency developing meansselectively connectable to said output circuit means to establish asecond frequency modulated carrier wave output from the transmitterinfluenced at said third frequency rate for a first time period and toterminate said third frequency thereafter; said receiver comprising amain load, first means responsive to said first frequency to pass asignal toward said main load, second means responsive to said secondfrequency to pass a signal toward said main load, third frequencyresponsive means and disabling means having an output and selectivelyconnectable to said main load with said disabling means operative todisable said main load in the absence of a received signal containingsaid third frequency, and said third frequency responsive means havingan output connected to terminate said disabling means output for atleast a second time period to enable said receiver to thus pass currentto said main load upon the presence of a received signal containing saidfirst, second and third frequencies and subsequently said first andsecond frequencies.
 14. A modulated carrier system as set forth in claim13, wherein said third frequency developing means includes a bridge Tfilter tuned to said third frequency.
 15. A modulated carrier system asset forth in claim 13, wherein said third frequency developing meansincludes a mechanically vibratable tuning fork.
 16. A modulated carriersystem as set forth in claim 13, wherein said third frequency responsivemeans includes a parallel resonant circuit resonant to said thirdfrequency.
 17. A modulated carrier system as set forth in claim 13,wherein said third frequency responsive means includes a mechanicallyvibratable tuning fork.